1. Field of the Invention
The present invention relates to a control method of filling step of an injection molding machine by controlling speed or pressure. More specifically, it relates to a filling step control method capable of eliminating fluctuation of injection speed during pressure control step, thus constantly obtaining molding products of good quality.
2. Description of Related Art
Conventionally, in an injection process including a filling step for filling molten resin material into a die cavity and a dwelling step for pressing the filled resin material to compensate contraction of the resin material filled in the die cavity on account of cooling, a preset filling pressure is set high so that an actual filling pressure does not exceed the preset filling pressure, injection speed is controlled at a multi-stage during the filling step and pressure to the resin filled inside the cavity is controlled in multi-stage during the dwelling step as shown in FIG. 3.
On the other hand, a low-pressure injection molding method without applying so high pressure has recently been used. In the low-pressure injection molding method, the preset filling pressure is set low as shown in FIG. 4. In initiating injection, the speed is controlled to be a preset injection speed, and subsequently, after the actual filling pressure is raised to reach approximately the preset injection pressure, the pressure is controlled so that the actual filling pressure does not exceed the preset filling pressure.
According to the low-pressure molding method, since an injection machine capable of filling at a high-pressure is not required, the cost for the machine itself can be reduced. Further, since the filling operation is conducted at the low-pressure, running cost for the injection machine can also be reduced.
Further, since the resin is injected at the low-pressure, the size of the die and peripherals can be reduced. Furthermore, energy required for operating the device can be reduced.
However, in the low-pressure injection molding, the injection speed was fluctuated as shown in lines A, B and C in FIG. 4 in an area for controlling the filling pressure (pressure control step) during the filling process.
The above phenomenon is thought to occur since the resin injection is totally dependent on pressure control, thus varying action of the resin inside the die.
On account of the fluctuation of the injection speed, quality of the molding product obtained for each shot has different qualities, thus not constantly obtaining quality products.
An object of the present invention is to overcome the above deficiency of the low-pressure injection molding where the injection speed was fluctuated for each shot during pressure control step and quality product was not obtained, and to provide a filling step control method of an injection molding machine capable of constantly obtaining quality products.
In order to achieve the above object, a filling step control method of an injection molding machine for filling a molten resin into a die cavity according to the present invention has the steps of: sampling in initiating the filling step for filling the molten resin into the die cavity by controlling an injection speed and, thereafter, by controlling a filling pressure while not exceeding a predetermined value; storing the injection speed over an entire filling process when a quality product can be stably obtained during the sampling step into a storage medium as a reference waveform; and after storing the reference waveform, molding for filling the molten resin into the die cavity by controlling the injection speed to follow the reference waveform over the entire filling step.
Accordingly, an injection speed waveform in the entire filling process when a quality product is obtained while conducting pressure control step by controlling pressure is stored in a controller of an injection molding machine as a reference waveform, and the molding step after storing the reference waveform is conducted following the reference waveform, thus capable of constantly molding quality products.
In the present invention, the sampling step and storing step for storing the reference waveform may preferably be conducted in advance, and the molding operation step may preferably be conducted using the reference waveform obtained during the sampling step and the storing step.
Accordingly, the reference waveform for the same molding die can be recycled for the same type dies even when the molding dies are frequently exchanged to mold variety of small number of molding products. Since repeated sampling process can be omitted, production efficiency can be improved.
Further, the injection speed may preferably be controlled during the molding by setting a plurality of injection speed values and switching positions of respective injection speed values.
In the present invention, the reference waveform may be represented using an approximate curve. However, the injection speed can be controlled with simpler arrangement by controlling the injection speed while setting the plurality of injection speed values and switching positions of respective injection speed values.
Further, the filling pressure may preferably be detected to raise an alarm when the filling pressure exceeds a predetermined upper preset pressure. Accordingly, safe molding operation is possible even when the pressure is not controlled during the molding operation.